The signal path between two telephones, involving a call other than a local one, requires amplification using a four-wire circuit. The cost and cabling required discourage extending a four-wire circuit to a subscriber's premise (i.e. Private Branch Exchange (PBX)) from the local exchange or Central Office (CO). For this reason, the four-wire circuits are coupled to two-wire circuits, using a device called a hybrid. Thus, when a PBX is connected to the CO through a Loop-Start (LS) Trunk Line, the hybrid couples the analog signal from the four-wire circuit (where incoming and outgoing signals are separated) to the two-wire circuit where the incoming and outgoing signals are combined.
Unfortunately, the hybrid is by nature a leaky device. As signals pass from the four-wire to the two-wire portion of the network, the energy in the four-wire section is reflected back, creating an echo of the signal. The intensity of the echo depends on how well the impedance is matched between both sides of the hybrid. The impedance of the two-wire circuit can vary wildly depending on factors including the line set-up in the CO equipment, the distance between CO and PBX, the electrical characteristics of the wire, etc. Provided that the total round-trip delay occurs within just a few milliseconds, the echo generates a sense that the call is ‘live’ by adding sidetone, thereby making a positive contribution to the quality of the call.
In cases where the total network delay exceeds 36 ms, however, the positive benefits disappear, and intrusive echo results. The actual amount of signal that is reflected back depends on how well the balance circuit of the hybrid matches the two-wire line. In the vast majority of cases, the match is poor, resulting in a considerable level of signal being reflected back.
It is known in the art to employ adaptive filtering to address hybrid echo cancellation. Normalized Least Mean Square (NLMS) adaptive filtering is one method, popular in echo cancellation, to address reflections in Voice-Over-IP (VOIP) systems.
Clearly, a well-matched four-wire circuit gives little echo and makes the echo-canceling task easier. However, the selection of a best set of matching impedance settings for a specific LS Trunk Line is currently very objective, mainly based on experience using trial and error. Such manual measurement consumes enormous human effort and time. Traditionally, a compromise setting is used that attempts to match a wide range of impedances, sacrificing overall ERL (echo return loss).
In the fields of DSL and ISDN (broadband access) techniques have been used to measure line characteristics, but for different purposes (e.g. to improve the DSL modem performance). Some prior art approaches are set forth in EP1357702, US2003173399 and US2003021391.